1. Field of the Invention
The present invention relates to a switch-controlling module and a method for controlling switches, and in particular to a synchronous rectifier controlling module and method for controlling a synchronous rectifier on a secondary side of power transformer.
2. Description of Related Art
The conduction loss of diode rectifier assembled by diodes or Schottky diodes contributes significantly to the overall power loss in a power converter (or power supply), especially in low output-voltage applications. The rectifier conduction loss is proportional to the product of its forward-voltage drop, and the forward conduction current. Metal-oxide-semiconductor field-effect transistors (MOSFETs) have advantages of low on-resistance, shorten responsible time, and high input resistance. Under certain current level, the forward-voltage drop of a synchronous rectifier assembled by MOSFETs can be lower than that of a diode rectifier, and consequently reduces the rectifier conduction loss. Hence, the synchronous rectifier is wildly used in power supplies.
Reference is made to FIG. 1, which is a circuit diagram of a conventional synchronous rectifier controlling module. The synchronous rectifier controlling module 4 is configured to control a synchronous rectifier on a secondary side of a power transformer T of a power converter (such as DC-to-DC power converter configured to convert a DC voltage into another DC voltage). The synchronous rectifier includes a first switch group SR1 and a second switch group SR2. As shown in FIG. 1, the first switch group SR1 includes four power switches MOS11, MOS12, MOS 13, and MOS14 electrically connected in parallel, and the second switch group SR2 includes four power switches MOS21, MOS22, MOS23, and MOS24 electrically connected in parallel. In particular, gates of each of the power switches MOS11˜MOS24 are electrically connected to the synchronous rectifier controlling module 4, drains thereof are electrically connected to a secondary winding of the power converter T, and sources thereof are electrically connected to ground.
In this manner, the synchronous rectifier controlling module 4 can control operating modes of the power switches MOS11˜MOS24. For example, all of the power switches MOS11˜MOS24 can be controlled to be always turned off when the synchronous rectifier controlling module 4 outputs a driving signal with continuously low voltage level lower than the gate threshold level of each of the power switches MOS11˜MOS24. In addition, the power switches MOS11˜MOS24 can be controlled to be turn on and off by following a waveform of a signal switching between two voltage levels outputted from the synchronous rectifier controlling module 4. In particular, the two voltage level mentioned above can be a high voltage level higher than or equal to a gate threshold level of each of the power switches MOS11˜MOS24 and a low voltage level lower than the gate threshold level each of the power switches MOS11˜MOS24.
The synchronous rectifier controlling module 4 is easily controlled and compact, however, whether the power converter is operated under light-load or heavy-load, all of the power switches MOS11˜MOS24 are simultaneously turned on and off by following the waveform of the driving signal outputted from the synchronous rectifier controlling module 4, which increasing the conduction loss of power converter.